U.S. Pat. No. 6,536,360 assigned to the present applicant discloses an air-curtain firebox designed to capture waste heat for useful applications. The heat is recovered from the side walls of the firebox, which radiate between 400 and 600 degrees Fahrenheit, using heat transfer panels to heat water circulating through tubing in the panels. The heated water is typically pumped to a radiator located in a building or greenhouse where air is heated to either warm a facility or provide process heat. The system taught by U.S. Pat. No. 6,536,360 has limitations. The captured heat is not converted to another form of energy, such as electricity. The system providing circulating water within the firebox walls requires maintenance, and leaks within the system can lead to significant downtime for repairs. This downtime may include burning downtime, not just power generation downtime.
U.S. Patent Application Publication No. 2012/0235423 A1 assigned to the present applicant discloses an air-curtain firebox system configured to generate electrical power from waste heat. In one embodiment, the system has heat transfer panels within side walls of a firebox for recovering heat in a manner similar to the firebox described in U.S. Pat. No. 6,536,360. In another embodiment, heat is recovered from an exhaust plume rising from the firebox near a side wall of the firebox opposite the air-curtain manifold. The heat from the exhaust plume is recovered by a heat recovery roof that extends partially over the firebox and contains heat transfer panels of the type described in U.S. Pat. No. 6,536,360. In the mentioned embodiments, the heat transfer panels carry a heat transfer medium, such as an environmentally benign refrigerant or a water solution, that changes from a liquid state to a gaseous state and rapidly expands when heated. The expanding gaseous heat transfer medium serves as a working fluid to drive a turbine or other type of expander of an electric generator to generate power. The heat transfer medium is condensed and cycled back through the heat transfer panels so the process can be repeated. Here again, internal leakage within the panels may lead to downtime when power cannot be generated and burning must be temporarily halted. In addition, the efficiency of the system suffers because the combustion heat fluctuates as biomass fuel is depleted and replenished, and there is no means to regulate heat transfer from the combustion plume to the heat transfer medium. For example, the heat transfer medium may expand faster than is optimal for driving the generator turbine, resulting in a loss of energy and a decrease in efficiency.
A further problem encountered in waste heat power generating systems of the prior art is the build-up of heat and pressure of the heat transfer medium that may occur if the generator goes offline. Prior art systems have dealt with this problem by providing an auxiliary bypass system having dump tanks, cooling means, and associated pumps and controls. This adds significant cost and complexity to the overall system.
Finally, many known systems for converting waste heat to electrical power are configured for use at a source of waste heat that remains at a fixed location, for example within a building where process equipment is housed. Those systems that are designed for travel to different locations, for example systems built directly into a portable air curtain firebox as discussed above, may only be used with that particular firebox, and cannot be used with another firebox or a fire pit.
What is needed is an improved heat capturing module and a power generating system that address the problems described above.